I. Field of the Invention
This invention relates to a prepreg and a composite. More particularly, this invention relates to a prepreg and a composite which are used for structures for which high strength and high elastic modulus, as well as high specific strength and high specific elastic modulus calculated by dividing the strength and elastic modulus with specific gravity, respectively, are demanded.
II. Description of the Related Art
Since advanced composites comprising reinforcing fibers and a matrix resin as indispensable constituents have heterogeneous structures, the physical properties in the direction of the fibers are largely different from those in the other directions. For example, the resistance to drop impact is governed by peeling strength between layers, so that it is not so improved even if the strength of the reinforcing fibers is increased. Composites having a matrix resin consisting essentially of a thermosetting resin have insufficient resistance to drop impact due to the poor toughness of the matrix resin. Thus, a number of methods have been proposed for improving the physical properties in the directions other than the reinforcing fibers, especially impact resistance.
Methods for increasing the toughness of the thermosetting resin per se include a method in which a polysulfone resin is added to an epoxy resin (Japanese Laid-open Patent Application (Kokai) No. 60-243113); and a method in which an aromatic oligomer is added to an epoxy resin (Japanese Laid-open Patent Application (Kokai) No. 61-228016). The impact resistance of the composite is allegedly improved by the increase in the toughness of the thermosetting resin.
Japanese Laid-open Patent Application (Kokai) No. 60-63229 discloses a method for increasing the impact resistance by providing an epoxy resin film modified with an elastomer between layers of fiber-reinforced prepregs.
U.S. Pat. No. 4,604,319 discloses that the impact resistance is improved by providing a thermoplastic resin films between layers of fiber-reinforced prepregs.
The present inventors disclosed in U.S. Pat. No. 5,028,478 a matrix resin containing particles made of a resin. Particularly, the present inventors disclosed that a composite having improved impact resistance while keeping the good tackiness and drapability of the prepreg can be obtained by localizing the resin particles in the surface of the prepreg.
U.S. Pat. No. 4,863,787 discloses that a composite having an improved impact resistance can be obtained by using a prepreg employing a matrix resin comprising an epoxy resin, reactive oligomer and elastomer particles having particle sizes of 10-75 .mu.m. It is described that a micro-phase separation is formed in the cured resin portion other than the elastomer particles.
EP-A2-0,377,194 discloses that a composite having an improved impact resistance can be obtained by using an epoxy resin containing polyimide particles soluble in epoxy resin with particle sizes of 2-35 .mu.m partially having a non-aromatic skeleton, such as aminophenyltrimethyl indane or the like. It is described that the soluble polyimide particles are dissolved in the interlayer zones of the composite during the curing.
Japanese Laid-open Patent Application (Kokai) No. 3-26750 discloses that a composite with excellent impact resistance can be obtained by employing a matrix resin comprising an epoxy resin, reactive polysulfone oligomer and a reactive elastomer, which contains resin particles comprising a reactive elastomer and an epoxy resin.
However, with these known methods, the increase in the impact resistance is insufficient, or the impact resistance is improved at the sacrifice of other properties such as thermal resistance and ease of handling.
More particularly, if the toughness of the resin is increased by incorporating a macromolecular thermoplastic resin such as polysulfone as disclosed in Japanese Laid-open Patent Application (Kokai) No. 60-243113, the viscosity of the resin composition is so high that it is difficult to impregnate the resin composition into the reinforcing fibers, and the tackiness and drapability of the prepreg are also deteriorated. In cases where an oligomer having lower molecular weight is added as disclosed in Japanese Laid-open Patent Application (Kokai) No. 61-228016, it is necessary to add the thermoplastic resin in a sufficient amount in order to attain a sufficient toughness of the resin. As a result, the viscosity of the resin composition is so high that it is difficult to impregnate the resin composition into the reinforcing fibers. Further, the more the amount of the thermoplastic resin, the poorer the solvent resistance of the cured material. Still further, even if the toughness of the resin per se is increased, the increase in the impact resistance of the composite is saturated. The maximum compressive strength after impact (compressive strength after impact is hereinafter referred to as "CAI") of a quasi-isotropic plate was given attained in the examples is only 46.1 ksi.
In cases where an independent film containing a thermosetting resin modified with an elastomer is used as disclosed in Japanese Laid-open Patent Application (Kokai) No. 60-63229, the more the content of the elastomer, the lower the thermal resistance. On the other hand, if the content of the elastomer is small, the increase in the impact resistance is very small. The maximum CAI indicating the impact resistance attained in the examples is only 48.3 ksi.
In cases where a thermoplastic resin film is used as disclosed in U.S. Pat. No. 4,604,319, although both of the thermal resistance and impact resistance are improved to some degree by employing a thermoplastic resin film having good thermal resistance, the tackiness and drapability which are advantageous features of thermosetting resins are impaired. Further, the solvent resistance of the composite is deteriorated due to the poor solvent resistance of the thermoplastic resin film, which is a common drawback of thermoplastic resins. The maximum CAI attained in the examples is only 51 ksi.
If the particles existing in the interlayer zones are made of an elastomer as disclosed in U.S. Pat. No. 4,863,787, the thickness of the interlayer zones is easily changed due to the changes in the curing conditions such as pressure and heating rate, so that the impact resistance is likely influenced by the curing conditions. Further, the existence of the elastomer decreases the thermal resistance of the composite. The maximum CAI attained in the examples is only 54.4 ksi.
In cases where a soluble polyimide is arranged in the interlayer zones as disclosed in EP-A2-0,377,194, the thickness of the interlayer zones is also fluctuated due to the fluctuation in the curing conditions, so that the impact resistance is influenced by the curing conditions. The maximum CAI attained in the examples is only 49.9 ksi.
Since Japanese Laid-open Patent Application (Kokai) No. 3-26750 does not describe the CAI in the examples, the impact resistances cannot be compared. It is thought that the elastomer in the particles reduce the thermal resistance.
On the other hand, U.S. Pat. No. 5,028,478 disclosed by the present inventors may be the closest prior art in the respect that a composite in which resin particles are localized in the interlayer zones is disclosed.
However, the composite disclosed in this U.S. patent is still imperfect because the toughness of the resin per se is insufficient. In the systems in which the impact resistance is promoted while keeping the thermal resistance, the maximum CAI attained in the examples is 53.3 ksi. Although the CAI may be increased to about 60 ksi within the scope of this U.S. patent by employing particles made of a thermoplastic resin having somewhat lower thermal resistance, the thermal resistance of the composite may be reduced in this case.